Notes: Pinkish with just the two dark stripes, and a hint of a dark stripe on the midline of the mantle suggest Lehmannia valentiana rather than Limax maximus. If so, it's often gregarious in compost bins, whereas the much larger L. maximus usually lives alone.

Brian Cambridgeshire said:
"The weak keel, strong dark stripes, pinkish background, and rather watery body are all typical of Spanish Slug. It's become quite widespread in gardens in the last 20 years, having previously been confined to heated greenhouses."http://www.ispot.org.uk/node/234455

It could be this but a juvenile Limax maximus seems more likely to me.